REINNERVATION OF STRATUM LUCIDUM BY HIPPOCAMPAL MOSSY FIBERS IS DEVELOPMENTALLY-REGULATED

Messy fibers from dentate gyrus granule cells establish synapses on CA 3 pyramidal neurons during the first 3 postnatal weeks in the rat. Mes sy fiber synapses are primarily restricted to the stratum lucidum. Whe n examined by Timm stain after 10-14 days in vitro, cultured hippocamp al slices from postnatal 4 rat pups show a similar messy fiber termina tion pattern in stratum lucidum. Thus, axon guidance cues used by mess y fibers in vivo appear to be preserved in these cultured slices. Thre e experimental manipulations were performed on hippocampal slice cultu res to examine whether the axon guidance cues used by messy fibers;ire developmentally regulated. First, massy fibers were transected on the day of culture or day 7 in vitro. Messy fibers transected on either d ay were able to reestablish their synaptic pattern in stratum lucidum of CA3, Second, dentates and hippocampi of same age or different age w ere co-cultured. Same age co-cultures P4 dentates to P4 hippocampi or P11 dentates to P11 hippocampi) showed good messy fiber reinnervation of stratum lucidum, as did different age co-cultures from P4 dentates to P11 hippocampi. However, P11 dentates to P4 hippocampi co-cultures showed little messy fiber reinnervation of stratum lucidum. Third, new P4 or P11 dentates were co-cultured onto hippocampal slices in which messy fibers had been allowed to degenerate, New messy fibers reinnerv ated these hippocampi, but did not reestablish their normal synaptic p attern in stratum lucidum. These three experimental manipulations sugg est that messy fiber axon guidance mechanisms are developmentally regu lated, and that existing messy fibers play a role in directing messy f iber reinnervation of stratum lucidum.